Your ebay link is not responding, but anyway, a G5 should have plenty of juice to run GB.

You really NEED an audio interface Mac 101: Audio Attachments
That article will give you an overview, a guitar should plug straight into it (providing it's an electric guitar of course).

Also, if you are going to record vocals you want the best possible microphone you can afford. If you are going to be recording mainly in a controlled environment, get a condenser mic like a rode NT2A otherwise a Shure Sm 58 is pretty much the stock standard dynamic vocal mic. I think an SM58 is around 200 though you can probably get one for less on ebay, any condenser mic worth owning will bee around 750 minimum.

The other tip is to trawl sites like HomeRecording.com there is a heap of tutorials and stuff on there. (sorry I am kind of assuming you are new to the whole recording thing, apologies if you are not) but you want to learn as much as you can about EQ, compression to make your music sound good. Capturing as good a signal as possible is important, but post production is just as important.

You might also consider a USB microphone. It might be a little bit more expensive, but you don't need any kind of interface to go from XLR to 1/8". You could record the vocals and mic an amp.

Otherwise, I would agree that SM58 is pretty much the standard vocal mic.

Do you know of a USB mic that would be ok for recording vocals and mic'ing an amp? My limited experience of usb mic's is that they sound pretty terrible for anything with any sort of dynamic range (guitar or amp).

The other thing you (JulieGirl) could/should look at is a ZOOM recorder H4n Handy Recorder

It records to an SD card which you can then drag and drop the resultant audio file into GB.

Apart from being able to stereo mic anything, it will take external mics and I think you can even use it as an audio interface. I know a guy who plugs an electric guitar straight into his and uses the amp modelling software that it comes with.

I've read some good things about the Samson G-Track USB microphone. It has gotten good reviews and all the sound clips I've heard have been pretty sweet. The mic does have line level inputs so you can plug your guitar directly in, but I'm not sure about miking an amp. I think the spec sheet says that it can handle up to 124 dB, but you should be alright as long as you're not trying to record a cranked marshall half-stack.

The microphone runs in the ballpark of $130, but you'll probably need to drop another $30 or so for the shock mount to put it on a mic stand.